1. Field of the Invention
The present invention relates generally to air supply control systems for internal combustion engines, and more particularly to a system for controlling air supply effected to an internal combustion engine by a plurality of superchargers accompanying with the internal combustion engine in substantially parallel with each other.
2. Description of the Prior Art
In the field of internal combustion engines used in vehicles, there has been proposed a so-called sequentially controlled supercharging system in which a couple of turbosuperchargers of primary and secondary are employed in an internal combustion engine in substantially parallel with each other and so controlled that only the primary turbosupercharger works for supercharging the engine when intake air mass flow in an intake passage of the engine is relatively small and both the primary and secondary turbosuperchargers work simultaneously for supercharging the engine when the intake air mass flow is relatively large, as disclosed in, for example, the Japanese utility model application published after examination under publication number 57-12177. In such a system, an exhaust cutoff valve is disposed in a portion of an exhaust passage of the engine through which exhaust gas is applied to a turbine of the secondary turbosupercharger and an intake air cutoff valve is also disposed in a portion of the intake passage of the engine through which air compressed by a blower of the secondary turbosupercharger is supplied to a combustion chamber of the engine, and each of the exhaust cutoff valve and the intake cutoff valve is controlled to be close and open so as to cause the primary and secondary turbosuperchargers in the aforementioned manner.
In connection with the control in operation of the primary and secondary turbosuperchargers, if the exhaust cutoff valve and the intake air cutoff valve are opened simultaneously when the engine is accelerated and thereby the operating condition of the engine is shifted into a situation in which the intake air mass flow is relatively large from a situation in which the intake air mass flow is relatively small, it takes a certain period of time after the exhaust and intake air cutoff valves are opened to drive the turbine of the secondary turbosupercharger to rotate at relatively high speed so as to make the supercharging operation of the secondary turbosupercharger effective, and therefore it is feared that air compressed by the primary turbosupercharger flows backward toward the turbosupercharger through the turbine thereof during the certain period of time after the exhaust and the intake air cutoff valves are opened. Accordingly, it has been proposed to cause the exhaust cutoff valve to be opened before the intake air cutoff valve is opened so that the secondary turbosupercharger is subjected to its preliminary rotation before it commences to work for supercharging the engine, for the purpose of avoiding the backward flow of compressed air toward the secondary turbosupercharger, and further to cause a intake air relief valve, which is disposed in an intake air relief passage provided to the intake passage for detouring the blower of the secondary turbosupercharger, to be open during the certain period of time, for the purpose of relieving air so that air surge in the intake passage is prevented from arising.
In the system previously proposed as described above, in the case where the engine is decelerated and thereby the operating condition of the engine is shifted into the situation in which the intake air mass flow is relatively small from the situation in which the intake air mass flow is relatively large, the intake air cutoff valve is closed when the intake air relief valve is opened and then the exhaust cutoff valve is closed with the intention of avoiding air surge arising in the intake passage. However, under such valve operations that the intake air cutoff valve is closed earlier then the exhaust cutoff valve, the blower of the secondary turbosupercharger continues to rotate for the period of time between a time point at which the intake air cutoff valve is closed and a following time point at which the exhaust cutoff valve is closed and further, with the force of inertia, for a certain period of time after the exhaust cutoff valve is closed. This results in that air surge in the intake passage is apt to be caused after the intake air cutoff valve is closed when the engine is decelerated.